The present invention relates to a bearing structure for a spindle motor, which employs hydrodynamic bearings as radial and thrust bearings so that the spindle motor is capable of rotating at high speed with minimal vibrations. More particularly, the present invention relates to a bearing structure which enables a spindle motor to exhibit an excellent rotating performance irrespective of the position of the motor when used and hence is suitable for a laser scanner motor, a hard disk driver (hereinafter referred to as simply "HDD"), etc.
With the developement of HDDs with a high storage capacity and low power consumption, a demand has arisen for improvement in the performance of spindle motors which are used to drive them so that these spindle motors are even more suitable therefor.
FIG. 8 is a partially sectioned elevational view of a conventional spindle motor for HDD. The spindle motor has a shaft support cylinder 22 in the center of a base 21. A group of stator coils 23 are secured to the outer periphery of the shaft support cylinder 22. A rotary shaft 25 is rotatably supported by the inner periphery of the shaft support cylinder 22 with ball bearings 24 interposed therebetween. The rotary shaft 25 has a rotor 27 secured to the upper end thereof, the rotor 27 being adapted to have hard disks fixedly mounted on the outer peripheral surface thereof. The rotor 27 has a group of rotor magnets 28 secured to the inner peripheral surface in opposing relation to the group of stator coils 23.
In the above-described spindle motor employing ball bearings, the magnitude of vibrations of the spindle motor depends on the internal clearances of the ball bearings. The magnitude of vibrations in the radial direction is substantially equal to the radial internal clearance of the ball bearings. Similarly, the magnitude of vibrations in the thrust direction is substantially equal to the thrust internal clearance of the ball bearings. Measures have been taken to reduce these internal clearances, for example, preloading the ball bearings. However, no satisfactory internal clearance value has heretofore been obtained, i.e., it has only been possible to achieve 0.5 microns or so in terms of the non-repeated component of the runout in the radial direction. In addition, preloading of ball bearings results in an increase in the required torque of the motor and hence works against to the desirous lowering in the power consumption of the HDD. Accordingly, as long as ball bearings such as those described above are used, it is in principle virtually impossible to further reduce the vibrations of the spindle motor.
In addition, the speed of motation of scanner motors for use in laser printers, for example, is increasing year by year, so that is has become difficult for conventional ball bearings to cope with the high rotational speed of these motors.
Under these circumstances, spindle motors which employ hydrodynamic bearings to realize a highly accurate rotatin performance have been porposed. FIG. 9 is a sectional view of a spindle motor which employs hydrodynamic bearings, according to an application by the present applicant in field advance of this application, now U.S. Pat. No. 4,998,033 . Referring to FIG. 9 a base 31 has a support shaft 32 stood on the central portion thereof. An annular thrust plate 33 is secured to the base 31, and a cylindrical radial bearing member 34 is concentrically secured to the support shaft 32. A plurality of equally spaced stator coils 35 are secured to the support shaft 32 above the cylindrical radial bearing member 34. On the other hand, a rotor 36 has a cap-shaped configuration. The top portion at the upper end of the rotor 36 is loosely fitted on the upper end portion of the support shaft 32. The rotor 36 has an annular member 37 secured to the lower end portion thereof, the annular member 37 having an L-shaped cross-sectional configuration. The lower end portion of the annular member 37 faces the thrust plate 33 to form a thrust hydrodynamic bearing having spiral grooves. The inner peripheral surface of the annular member 37 faces the radial bearing member 34 to form a radial hydrodynamic bearing having herringbone-shaped grooves.
A plurality of equally spaced rotor magnets 38 are secured to the inner periphery of the rotor 36 in opposing relation to the stator coils 35. As the stator coils 35 are sequentially supplied with an electric current, the rotor 36 having the rotor magnets 38 begins to rotate and consequently a pneumatic pressure is generated between the upper surface of the thrust plate 33 and the lower surface of the annular member 37, thus forming a pneumatic thrust bearing. Similarly, a pneumatic pressure is generated between the outer peripheral surface of the radial bearing member 34 and the inner peripheral surface of the annular member 37, thus forming a penumatic radial bearing. Since the annular member 37 is supported without being in solid contact with the associated members, the spindle motor is capable of smoothly rotating at high speed in contrast to the prior art that employs ball bearings.
However, the above-described spindle motor still suffers from the problem that, when it is operated in a horizontal position (i.e., in a direction in which the direction of gravity is perpendicular to the shaft of the motor), a moment in the radial direction is generated due to the gravity of the rotor, causing the radial bearing to be inclined with respect to the support shaft, which results in an increase in the imbalance of radial magnetic force acting between the rotor magnets and the stator coils, and in this state the rotor is brought into local contact with the bearing.
Further, since the bearings are comprised of a number of discrete members, it is difficult to achieve the required perpendicularity at the time of assembly.
In addition to the problem described above, when the conventional spindle motors that employ hydrodynamic bearings are used in a horizontal position, the following problems are experienced:
First, the magnitude of vibrations during rotation is large.
Secondly, when two discrete bearings are used, it is difficult to align them concentrically at the time of assembly. In particular, since the clearance between a movable piece and a fixed piece of a radial bearing is on the order of microns, it is difficult to align them concentrically during the manufacturing process. In the case of a bearing structure in which two thrust bearings are provided separately from each other, it is difficult to adjust the relative position of the two thrust bearings. In addition, since the thrust collar of a thrust bearing is produced so that the parallelism is within several microns and it is necessary to hold down the parallelism to about 1 micron when it is assembled, it is very difficult to produce it.
Thirdly, in a radial gap type spindle motor such as that shown in FIG. 9, a moment is generated due to the imbalance of radial magnetic force acting between the rotor magnet group and the stator coil group, causing the axis of the rotor to be inclined with respect to the support shaft, which results in an increase in the starting torque because of local contact of the dynamic pressure surfaces. During rotation, unstable radial magnetic force, which is added to the dynamic pressure, causes whirling of the rotor and therefore makes it impossible to obtain a satisfactory operating condition.
In view of the above-described problems, it is an object of the present invention to provide a bearing structure suitable for use in a spindle motor, which employs hydrodynamic bearings to improve the high-speed rotating performance and minimize the vibration irrespective of the position of the motor when used.